Switch to llvm20/clang20
- Fixes failure to build from source with llvm21/clang21.
- Upstream issue https://github.com/dotnet/runtime/issues/119706.
Due to an influx of spam, we have had to temporarily disable account registrations. Please write an email to accountsupport@archlinux.org, with your desired username, if you want to get access. Sorry for the inconvenience.